Sean O'Callaghan assisted a number of terror victims seeking justice after he turned his back on the IRA

The 63-year-old Co Kerry native was jailed in the 1990s after walking into a police station in England and admitting two IRA murders. He was freed under a royal prerogative of mercy in 1997.

Following his release, he used his expert knowledge of the republican movement to help a number of bodies better understand the IRA’s methods and motives – and assisted a number of terror victims in their quest for justice.

Mr O’Callaghan worked as an unpaid advisor to former first minister David Trimble during the Good Friday Agreement negotiations, and supported relatives bereaved in the 1998 Omagh bomb in taking a successful landmark civil case against four republicans.

Having served a number of prison sentences for his involvement in IRA terrorism, Sean O'Callaghan became vehemently opposed to republican violence

Earlier this year, Mr O’Callaghan named two senior IRA men as being responsible for the sectarian massacre of 10 Protestant workmen at Kingsmills in south Armagh while giving evidence at an inquest examining the 1976 murders.

Historian and author Ruth Dudley Edwards was a close friend, describing Mr O’Callaghan as “a man of exceptional ability and courage”.

She said: “He and I were very close friends for more than 20 years, and, like all his friends, I loved him very much and owe him a great deal for his insights, his wise advice, the depth of his knowledge of politics, history and the human condition.

“When he was writing his book [The Informer] he was absolutely determined to get it right and to tell the truth. Nobody could prove him wrong on anything. He had a great regard for truth ... he had a great sense of history.”

“He joined the IRA at 15 thinking he was a resistance fighter, but then he realised that he was in a squalid sectarian war and that the IRA was far more interested in killing Protestant neighbours than killing British soldiers.”

William Matchett, former Special Branch officer and author of Secret Victory: The Intelligence War that Beat the IRA, also got to know him well and considered the former terrorist “a good friend”.

Mr Matchett said: “He was haunted by what he did for the Provisionals and annoyed at how he was duped by the romantic notion of ‘armed struggle.’ He did his best to atone for his bad deeds, but I do not think he ever satisfied himself that this was enough. Sean had a conscience. His regret, remorse and sorrow for what he did were genuine.”

Mr Matchett added: “He saw the Provisional IRA as sectarian and controlled by bigots from the ‘north’ who constantly postured to stay in power. The last time I talked to him he related that some attacks were carried out purely to strengthen the hard line credentials of a senior figure so that a rival could not take his position.

“He was a proud Irishman. But of course, the ‘informer’ status provoked the anger of all those who subscribe to the Provo brand of republicanism. To them, he is a hate figure.

“I enjoyed his company and appreciated his advice. I will miss him deeply. My sincere sympathy to his family and friends. Sean O’Callaghan was a son of Ireland and one of its true patriots.”

Michael Gallagher, whose son Aiden died in the Omagh bomb, also passed on his condolences.

“My memories of Sean will always be good memories,” he said.

“He assisted in every way possible to make our civil action happen – to help the Omagh families get some form of justice. He was somebody who obviously had a past, but he very courageously decided to change and he saved many lives,” Mr Gallagher added.

Toby Harnden, author of Bandit Country, also paid tribute to the “wit and kindness” of Mr O’Callaghan.

“I first got to know Sean when I visited him regularly in Maghaberry from 1996 until his release from jail and he has been a true friend since. He was a complex, intelligent and very brave man. I will miss his thoughtfulness, wit and kindness,” he said.A spokesman for the Irish Department of Foreign Affairs said: “The department is aware of the case and stands ready to provide consular assistance if requested.”